This invention relates to improvements in a control apparatus for an A-C elevator which controls the elevator to be driven by an induction motor.
A system which employs an induction motor as an electric motor for driving the cage of an elevator, wherein the motor is subjected to a slip frequency control, whereby the torque of the motor is controlled so as to run the cage is illustrated in FIGS. 5 and 6.
In FIG. 5, numeral 1 designates a three-phase A-C power source terminal, numeral 2 an electromagnetic contactor contact which is connected to the terminal 1 and which is closed when starting a cage 11 to be described later and is opened when stopping it, and numeral 3 a converter which is constructed of a rectifier circuit for rectifying a three-phase A-C voltage into direct current through the contact 2. Numeral 4 indicates a smoothing capacitor which is connected to the D-C side of the converter 3, numeral 5 a resistor which has one end thereof connected to one end of the smoothing capacitor 4, and numeral 6 a transistor which is connected to the other end of the resistor 5 and the other end of the smoothing capacitor 4. An inverter 7 well known as a pulse width modulation system is connected to both the ends of the smoothing capacitor 4, is constructed of transistors and diodes, and inverts a fixed D-C voltage into alternating current of any desired voltage and any desired frequency. A three-phase induction motor 8 is driven by the inverter 7, a driving sheave 9 for a hoist is driven by the motor 8, a main rope 10 is wound round the sheave 9, the cage 11 and a counterweight 12 are respectively coupled to both the ends of the main rope 10, and a tachometer generator 13 is coupled to the motor 8 and produces a speed signal 13a indicative of the rotational speed of the motor 8. Shown at numeral 14 is a speed command signal. An adder 15 outputs the deviation signal between the speed command signal 14 and the speed signal 13a. Numeral 16 indicates a compensation element which is connected to the adder 15 in order to better the response of a speed control loop, and which has a transfer function G(s). Shown at symbol 16a is a slip frequency command signal which is the output of the compensation element 16. An adder 17 outputs the added value between the slip frequency command signal 16a and the speed signal 13a. A voltage command generator 18 produces a voltage command signal 18a on the basis of the output of the adder 17, and a frequency command generator 19 similarly produces a frequency command signal 19a. An inverter control device 20 controls the output voltage and output frequency of the inverter 7 on the basis of the voltage command signal 18a and the frequency command signal 19a.
In such an arrangement, the compensation element 16 outputs the slip frequency command signal 16a which corresponds to the deviation between the speed command signal 14 and the speed signal 13a. This slip frequency command signal 16a corresponds to a torque command signal, and the speed signal 13a is added thereto by the adder 17, whereby the voltage command signal 18a and the frequency command signal 19a are determined so as to satisfy the relationship in which the ratio of a voltage/ a frequency becomes substantially constant. Using these command values, the control device 20 performs the switching control of the elements of the inverter 7 (also those of the converter 3 in some cases when the converter 3 is constructed of thyristors or the likes) and causes the motor 8 to generate the torque corresponding to the slip frequency command signal 16a. In this way, the motor 8 is started to run the cage 11, the speed of which is automatically controlled at high precision.
FIG. 6 shows the curve 21 of the torque versus the rotational speed of the motor 8. Here, n.sub.O denotes a synchrongus speed, which is an operating point in the case where the motor 8 rotates at a rotational speed corresponding to the output frequency of the inverter 7. In general, an induction machine produces near the synchrongus speed n.sub.O a torque proportional to a slip frequency (corresponding to, for example, n.sub.O -n.sub.1 or n.sub.O -n.sub.2). Accordingly, when the magnitude n.sub.O -n.sub.1 corresponding to (slip x frequency) is controlled, the torque can be controlled.
In case of the elevator, however, when the cage 11 is to be run down with a heavy load and at a fixed speed for the purpose of the slowdown stoppage thereof, the motor 8 needs to generate a braking force. In the illustration of FIG. 6, this corresponds to a case where a required braking torque is T.sub.2. In this case, the rotational speed n.sub.2 of the motor 8 becomes higher than the synchrongus speed n.sub.O. In such an operation, mechanical energy is converted into electrical energy through the motor 8, and regenerative power is returned to the D-C side through the inverter 7. Therefore, a D-C side voltage rises and might destroy the elements within the inverter 7. The resistor 5 protects the elements against this voltage, namely, the regenerative power is consumed in the resistor 5 owing to the turngn of the transistor 6. There is also a system wherein the resistor 5 is replaced with an inverter for regeneration, which returns the regenerative power to the power source side.
In any case, however, a device for processing the regenerative power becomes expensive, and the control apparatus becomes large in size.
Intended to solve this disadvantage is a method wherein regenerative power is consumed within a motor as disclosed in the official gazettes of Japanese Patent Applications Laid-open No. 59-17879 and No. 58-36866.
More specifically, referring to FIG. 7, numeral 101 designates a rectifier which converts alternating current into direct current, numeral 102 a capacitor which smooths the direct current, numeral 103 a well-known inverter device of the pulse width modulation system which switches D-C power by means of transistors and inverts it into an A-C output of variable voltage and variable frequency, numeral 104 an induction motor which is driven by the inverter 103, numeral 105 a driving sheave for a hoist which is driven by the motor 104, numeral 106 a main rope which is wound round the sheave 105, and numerals 107 and 108 a cage and a counterweight which are respectively coupled to both the ends of the main rope 106.
Further, numeral 109 indicates a tachometer generator which is coupled to the induction motor 104 and which produces an actual angular velocity signal .omega..sub.r indicative of the rotational speed of the motor, numera 110 a speed control calculation device which generates a torque command T* on the basis of the difference between an angular velocity command signal .omega..sub.p and the actual angular velocity signal .omega..sub.r, numeral 111 a frequency calculation device which calculates a feed frequency so that regenerative power may be entirely consumed within the motor 104 when the motor is to generate a braking torque, numeral 112 a current value calculation device which calculates the magnitude of a required current I* on the basis of the torque command T* (in this case, the braking torque) from the speed control calculation device 110, numeral 113 a current command generator which calculates the instantaneous value commands i.sub.u *, i.sub.v * and i.sub.w * of the currents of respective phases on the basis of the magnitude of the current I* and the calculated frequency value .omega.*, and numeral 114 a pulse width modulation (hereinbelow, termed "PWM") switching circuit which compares the current commands with actual currents detected by a current detector 115 and which brings the actual currents closer to the command values. The current detector 115 detects the currents of the two phases of the U-phase and the V-phase, and the current of the W-phase is obtained as the difference between the detected currents.
Next, before the description of the operation of the prior-art example, the principle of this prior-art example will be elucidated with reference to FIGS. 8 and 9. FIG. 8 shows a simplified equivalent circuit for a single phase with note taken of only resistance components within the induction motor, while FIG. 9 graphically shows electric power generated by the motor and electric power consumed within the motor, versus a slip evaluated from the equivalent circuit. Referring to these figures, a condition for fully consuming regenerative power within the motor is to satisfy the following equations: ##EQU1## Here, r.sub.1 and r.sub.2 indicates the primary and secondary resistance components of the motor respectively, and S the slip thereof. That is, when the slip meets Eq. (2), the regenerative power is entirely consumed within the motor, and when the slip becomes greater than the magnitude (i.e. becomes smaller in the absolute value thereof), electric power is returned to the D.C. side. On the other hand, the torque T is expressed by: ##EQU2## Here, .omega..sub.r indicates the actual rotational angular velocity of the motor, and .omega. the drive frequency thereof. As seen from Eq. (4), when the slip S satisfies Eq. (2) and assumes the constant value, the torque T is proportional to the square of the current I and is inversely proportional to the drive frequency.
On the basis of these conditions, in the arrangement of FIG. 7, the motor 104 shall be braked when the torque command T* which is output from the speed control calculation device 110 on the basis of the difference between the actual angular velocity .omega..sub.r and the ideal angular velocity command .omega..sub.p of the motor corresponding to the ideal speed curve of the elevator has become a braking torque command. Naturally, these operations are executed also through the calculation of S/.omega. by a microcomputer or the like. In this case, the change-over from the power running to the braking can be readily effected by changing-over S/.omega. calculation methods depending upon the sign of the torque command T* .
Here, when the torque command T* has become the braking torque command, the magnitude of the current meeting Eq. (4) is evaluated by the current value calculation device 106. In addition, the frequency at which the slip fulfills Eq. (2) is generated by the frequency calculation device 111. The current magnitude and the frequency with which the motor 104 is driven so as to prevent the generation of the regenerative power while producing the necessary braking torque are evaluated by these calculations. Thereafter, the current command values of the respective phases are found by the current command generator 113, they are delivered to the PWM switching circuit 114 and compared with the actual currents therein, the outputs of the switching circuit operate the inverter 103, and currents are fed to the motor 104. Thus, the motor 104 runs the cage 107 in accordance with the ideal speed curve of the elevator while generating the necessary braking torque.
The prior-art apparatus adopts either of two methods wherein in generating the braking torque, the regenerative power is efficiently generated and is entirely consumed by a resistor (not shown) disposed on the D-C side or wherein the regenerative power is entirely consumed within the motor as described above. With the former method, the capacity of the resistor is large and the apparatus becomes expensive. On the other hand, the method by which the regenerative power is consumed within the motor needs to enlarge the absolute value of the slip. Accordingly, the current value becomes large as understood from Eq. (4), the current capacity of the inverter 103 increases, and the apparatus rather becomes expensive. Besides, when the electric power to be consumed within the motor is high, the motor produces much heat, so that the motor will be inevitably enlarged in size or will degrate earlier.